AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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1
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Medical Physicist’s Testing
Procedures for MRI
Equipment Evaluation
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Geoffrey D. Clarke, Ph.D.
clarkeg@uthscsa.edu
ACMP Annual Meeting
May, 2010
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Overview
• Phantoms for ACR MRI Accreditation
• Geometric Distortion, RF Receiver
Bandwidth and Magnetic Field
Homogeneity
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• Resolution, Signal-to-Noise Ratio &
Signal Intensity Uniformity
• Slice Thickness, RF Transmission &
Ghosting Artifacts
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Phantoms for
ACR MRI Accreditation
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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ACR
Magnetic
Resonance
Imaging
Quality
Control
Manual
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(rev. 2004)
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5
•
MRI Accreditation
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Standard Phantom Design Goals
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Easy to Use
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– Multiple inserts
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– Not too bulky
– Applicable to all
MRI systems
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• Moderately
Priced
– $1050
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ACR Standard MRI Phantoms
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ACR MRI Phantoms Produced
Number of Phantoms Produced
6
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8000
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7000
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6000
~1000/yr
5000
4000
3000
2000
1000
0
J- J- F- A- M- S- A- N- M- D- J- J- J- F- S- M- O- A- N- M- D- J- J97 97 98 98 99 99 00 00 01 01 02 03 03 04 04 05 05 06 06 07 07 08 09
Month-Year
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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MRAP Standard Phantom Models
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S/N 2256
S/N 2857
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8
ACR MRAP Small Phantom
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Small version
version of standard ACR MRAP
MRAP phantom
phantom –– designed
designed for
use is
is clinics
clinics that
that have dedicated
dedicated extremity
extremity imagers that allow
allow
application
application for
for ACR Accreditation of
of knee
knee module only.
Phantom
Phantom Test Guidance
Guidance for
for Use
Use of
of the Small
Small MRI
MRI Phantom for the
the ACR
ACR MRAP
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9
MRAP Small Phantom Specs
• inside length 100 mm
• inside diam. 100 mm
Saggital image
image show
show positions for
seven slices acquired with small
ACR MRAP phantom
• filled with a solution of NiCl
& NaCl
– 10 mM NiCl2 and 0.45% by
weight aqueous NaCl.
• A separate vial is filled with
20 mM NiCl2 but no
aqueous NaCl.
Phantom
Phantom Test Guidance
Guidance for
for Use
Use of
of the Small
Small MRI
MRI Phantom for the
the ACR
ACR MRAP
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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MRAP Small Phantom Specs
10
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•
•
•
•
Slices are nominally 5mm with 3 mm gaps
12 cm FOV; matrix size is nominally 192 (fe) × 152 (pe)
High contrast resolution arrays are 0.9mm, 0.8mm & 0.7mm
Crossed wedges have 45oo slopes - the bar length difference
is twice the actual slice displacement error
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11
MRI Phantoms: General Features
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– Nonsignalproducing container
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– Proton density
similar to water
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– Shorten T1: NiCl &
CuSO4
– Mimic Conductivity
of tissues: NaCl
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12
Phantoms Developed by Users
Uniform
Spherical
Phantoms
Agarose
Gel
Phantom
with Inserts
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Geometric Distortion, RF
Receiver Bandwidth and
Magnetic Field Homogeneity
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14
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Geometric Accuracy
• Measure
distance
along main
axes of
phantom
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• Compare
with known
values
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15
Air Bubble
• When air bubble
is in phantom,
geometric
distortion
measurement
may have to be
taken along
diagonal.
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
16
Watch Out for Distortion Correction
Geometric
Distortion
correction
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RF Filter
Roll-off
• Distortion correction can warp the FOV
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17
Gradient Correction
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• If gradients
are inherently
non-linear
gradient
correction
may be
applied
before
images are
displayed
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18
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Magnetic Field Homogeneity
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Ideal Homogeneity Good Homogeneity Poor Homogeneity
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FWHM
FWHM
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FWHM
ωoo
Denotes
Denotes aa totally
totally
uniform
uniform magnetic
magnetic
field.
field.
All
All signal is at
at
resonant
frequency,
frequency, ωo.
o.
ωoo
ωoo
Fourier transform of
signal
signal produces a
Lorentzian peak
peak in
in
well-shimmed
well
well-shimmed
magnet
Magnet
Magnet firld
homogeneity
homogeneity can
can be
be
characterized
characterized using
FWHM
FWHM of resonance
resonance
peak
peak
AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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19
ACR Phantom – Phase Maps
Axial
Axial
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Phase and
Unwrapped
Phase Images
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Sagittal
Sagittal
180
180oo phase
phase variation
Best homogeneity is
in center, edges of
phantom degrade
uniformity of B-field in
phantom
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20
Magnetic Field Homogeneity
spectral line widths
phase-difference map
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Data from MRI System Manufacturer’s Phantom
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21
Magnetic Field Homogeneity
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• Overall, the phase mapping technique
provides the best mechanism for
evaluating field homogeneity.
• Phase-maps in several planes can be
obtained to determine the spherical
harmonic coefficients and allows a means
of “shimming” the magnet.
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
Estimating
Receiver Bandwidth
22
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Distance between Fat & Water represents
the amount of the field of view which
spans 3.5 ppm of the resonant frequency:
Bandwidth (Hz) =
(
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)
FOV (mm) × 3.5 ×10-6 × proton frequency (Hz)
chemical shift (mm)
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Image with Small Bandwidth
• Large
chemical
shift
indicates
small BW
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Small Bandwidth = Large Distortion
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T1W
T2W
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• Smaller BW on T2W image leads to increased
image distortion
AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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25
Spherical Homogeneity Phantom
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coronal
saggital
axial
Photo of homogeneity phantom
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26
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Bandwidth-Difference Method
MFH ( x, y ) =
γ
BW1 ⋅ BW2 (d1 − d 2 )
⋅ FOV ⋅ ( BW2 − BW1 )
• The MFH is measured from
the change in distance
between landmarks in the
phantoms between
the
two bandwidths.
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d
• FOV = field of view in m
• γ = γ/2π = 42,567 Hz/mT
Chen
Chen HH
HH et
et al.
al. Medical
Medical Physics,
Physics, 2006,
2006, 33(11):
33(11): 4299-4306.
4299-4306.
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Slide
Optimization of Parameters
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Minimum ppm Measured
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1.2
BW2 =
10000 Hz
1
minimum ppm
27
0.8
BW2 =
25000 Hz
FOV =
330 mm
0.6
BW2 =
50000 Hz
0.4
0.2
0
0
5000
10000
BW1 (Hz)
15000
d1-d2 =
1 mm
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
28
Magnetic Field Homogeneity
• For some systems, service personnel
may provide use of phase-mapping
acquisition and analysis tools.
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• Filmed copy of vendor’s final
homogeneity map and shim coefficients
is useful for documenting initial
conditions and establishing a baseline.
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Resolution,
Signal-to-Noise Ratio
& Signal Intensity Uniformity
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30
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Spatial Resolution Matrix:
Registration with Phantom
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PARTIAL VOLUME ARTIFACT
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Resolution
Holes
Image
Matrix
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
Total Point Spread Function
31
Due
mainly to T2**
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∆x
∆y
Due mainly
to matrix size and FOV
Total PSF
Instrumental PSF
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Slide
32
Excessive Truncation Artifacts
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If receiver
bandwidth is
set
too
low, images
become
prone to
major
truncation
artifacts.
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33
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ACR MRI Phantom
Chemical Shift Insert
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Slice Thickness
Fat
H2O
1.1 mm
1.0 mm 0.90 mm
Chemical
Shift (mm)
Resolution Insert
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
Slide
34
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Truncation (Gibbs) Artifact
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• Appears as
lines of
alternating
darkness and
brightness
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• Occurs in
both read-out
and phaseencoding
directions
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35
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Fourier Truncation Artifact
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Fourier Convolution Theorem:
ρ DFT =
Lx / 2
∞
− Lx / 2
−∞
ρ (τ )h( x − τ )dτ = ρ (t )h( x − τ )dτ
or ρ DFT = ρ ( x ) ∗ h( x)
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The convolution kernel, h(x), is oscillatory and
merges closely spaced features together
gives rise to spurious ringing
This effect is most pronounced where the image exhibits a
step discontinuity of signal intensity
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36
Truncation Errors (Gibbs Artifact)
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Fourier
Convolution
True image
reconstruction,
function, ρ(x) kernel, h(x)
ρDFT(x)
Truncation Artifact is
prominent in ACR slice
thickness insert
AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
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Gibbs Artifacts
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“Scalloping”
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LCD perception
• Use of edge enhancement filtering
enhances truncation artifacts
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38
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Methods for Reducing
Truncation (Gibbs) Artifacts
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• Use smoothing filter
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– Will cause high contrast spatial
resolution to be degraded
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• Use large matrix size
• Don’t have regions with abrupt signal
intensity transitions in the phantom
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39
Low Contrast Detectability
Four sets of plastic membranes with holes
1.5 mm to 7 mm in diameter
1.4% Contrast
Contrast Detectability
Detectability
2.4%
2.4% Contrast
Contrast Detectability
Detectability
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Slice #8
Slice #9
AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
Low Contrast Detectability
40
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Partial volume effect used to obtain contrast
from membranes of variable slice thickness
3.7% Contrast
Contrast Detectability
5.1%
Contrast Detectability
Detectability
5.1% Contrast
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Slice #10
Slice #11
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Slide
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LCD and Signal-to-Noise
Total Number of Spokes
41
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40
40
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35
35
+SD
30
30
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25
25
20
20
15
15
10
10
55
00
00
50
50
100
100
150
150
200
200
250
250
300
300
350
350
Signal-to-Noise Ratio
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Slide
Truncation Errors
42
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True image
function, ρ(x)
Fourier
Convolution
kernel, h(x) reconstruction, ρDFT(x)
• Truncation Artifact is
prominent in ACR slice
thickness insert
• Occurs in both read-out and
read
read-out
phase-encoding directions
phase
phase-encoding
AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
43
Radio Frequency Coil Checks
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• Volume coils
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– Signal-to-noise ratio
– Percent integral uniformity
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– Percent signal ghosting
• Surface Coils only Maximum SNR Tests
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44
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Image Uniformity Reflects RF
Field Distribution
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Slotted Resonator
Birdcage with Eight
Straight Elements
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45
Uniformity Pattern
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Birdcage Coil
High Field
Solenoid Coil
Low Field
AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
Image Uniformity
46
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Big ROI =
195 cm2
(19,500
mm2)
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Max Signal
Small ROI’s
= 100 mm2
Min Signal
Signal
ACR phantom - Slice #7
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47
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Volume Coil Data
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% Image Uniformity
Max Signal
Min Signal
Signal-to-Noise
Mean Signal
SD of Background Signal
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Ghost Signal
Mean Signal
Background Signal
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Percent Signal
Ghosting
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Surface Coil Data
Maximum Signal-toNoise
Maximum signal
SD of Background Signal
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48
Volume RF Coil Measurements
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ACR
Phantom
Slice #7
AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
Image Intensity Uniformity
49
• Performance criteria: PIU ≥ 87.5%
except 3T (82%)
(high − low)
percent integral uniformity = 100 × 1 −
(high + low)
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• Measurement Considerations:
• Display may not show signal values
• Display may not allow user to set signal display
level
• There may not be a well-defined high/low
intensity level
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Slide
Uniformity Patterns
50
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Birdcage Coil
High Field
Solenoid Coil
Low Field
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Slide
Uniformity at 3 Tesla
51
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1.5T
1.5T
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33 T
Requirement for percent integral
uniformity (PIU) for a 3T MRI
system is equal to or greater
than 82%.
• B1 field maps in a conductive saline phantom
(18 cm diameter)
RL
RL Greenman et
et al.
al. JMRI
JMRI 2003, 17(6):
17(6): 648-655
648-655
AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
52
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Surface RF Coil Measurements
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53
Ghosting is Nonspecific
• Instability in MRI signal from pulse to pulse
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• Phantom motion
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• Loose connections or bad cable
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• Partial failure of radio frequency coils
• Pulse sequence calibration error
– Eddy currents in Fast Spin Echo series
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Slide
54
Ghosting
• Small amounts
of ghosting may
not be above
ACR T1W GR
limits
• May still cause
failure on other
tests
• Ghosting on
T2W images
should be noted
& corrected
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
55
Ghosting & LCD
• Ghosting that
doesn’t cause
a failure of
itself may
obscure
otherwise
visible LCD
spokes
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Slide
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56
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Slice Thickness,
& RF Transmission
Artifacts
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Slide
57
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RF Noise/Leaks/ Spikes
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Single
frequency
artifact
shows up as
liner in
image.
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
58
DC-Offset Artifacts
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• MR Signal rides
on top of DC
offset
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• Leads to zero
frequency
artifacts
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• Not important if
off to side
• Can be
controlled
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Slide
59
Zipper Artifact
• Due to
transverse
magnetization
created by
180o refocusing
pulse.
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• Can be
eliminated with
crusher
gradients.
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Slide
60
Zipper in Sagittal Image
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• May be due to
large slice
thickness
• Inadequate
suppression of
stimulated
echo artifact
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
61
RF Clipping / Shine Through
• Due to miscalibration of
RF receiver
gain or
attenuation
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• Clipping of RF
by ADC
introduces
spurious high
frequencies
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Slide
62
Susceptibility Artifacts
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Small
inclusions
in LCD
insert can
hamper
test
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Slide
63
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Susceptibility Artifacts
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Flaw is in
phantom,
however
measurement
can still be
completed
without
compromising
accuracy
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AbstractID: 14612 Title: Medical Physicist’s Testing Procedures for MRI Equipment
Evaluation
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Slide
64
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Air in Slice Thickness Insert
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Flaw is in
phantom,
however
measurement
can still be
completed
without
compromising
accuracy
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Slide
65
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Slice Thickness Profiles
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Hitachi Slice Profile #1
6000.00
Pixal Intensity
5000.00
___________________________________
4000.00
3000.00
2000.00
___________________________________
1000.00
0.00
0
10
20
30
40
50
60
70
80
90
___________________________________
Pixel number
Hitachi Slice Profile #2
250.00
Pixal Intensity
200.00
150.00
100.00
50.00
0.00
0
20
40
60
80
100
Pixel number
• Hitachi Aris slice profiles may look strange on certain
models and/or versions of software
Slide
66
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Summary
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• The ACR phantom is not adequate for
all QC tests – the QMP/MRS must use
other phantoms & methods
• All radiofrequency coils must be tested
in every mode of operation used
clinically.
• It is advisable to use coil
manufacturer’
’s phantoms and tests
manufacturer
manufacturer’s
where available.
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